1. Field of the Invention
The present invention relates to a vehicle lighting device, and in particular, to a vehicle lighting device in which a light emitting element is employed as a light source.
2. Description of the Related Art
Conventionally a load that is operable by a current, for example, a light emitting element such as an LED (Light Emitting Diode) having a feature that a luminous intensity of light changes depending on a value of a current is employed as a light source of a vehicle lighting device. The vehicle lighting device with such a load has a function that a current flowing in the load is detected, which is converted to a voltage, and the current flowing in the load is controlled to be constant based upon this voltage. An explanation will be made of a conventional example of the vehicle lighting device having such a function with reference to FIG. 5 to FIG. 8.
A conventional, basic vehicle lighting device depicted in FIG. 5 comprises a power conversion component 1, an LED 2 and an output control component 3, and an example of a vehicle lighting device shown in FIG. 6 further comprises an output current setting device 7 in addition to these components. The power conversion component 1 converts a direct-current power that is supplied through an input terminal +V and a ground terminal GND, and supplies an output current 5 that is outputted by the conversion to the LED 2. In addition, the power conversion component 1 detects the output current 5, and outputs an output current signal 4 based upon this detection.
The LED 2 emits a luminous flux that is set from a plurality of ranks in accordance with a plurality of predetermined luminous fluxes. The output current 5 of the set current is supplied to the LED 2 in such a manner as to emit the above set luminous flux. The output current setting device 7 depicted in FIG. 6 outputs a set output current setting signal 8 to the output control component 3. That is, the output current setting device 7 outputs the output current setting signal 8 to the output control component 3 such that the output current 5 becomes the above set current and the LEI) 2 emits the set luminous flux.
The output control component 3 comprises a frequency generating component 3a, an output current detecting component 3b, and an output adjusting-signal generating component 3c. The frequency generating component 3a generates a frequency signal of a predetermined, fixed frequency, for example, a triangle wave.
The output current signal 4 and the output current setting signal 8 are inputted to the output current detecting component 3b, which sets a reference voltage as a reference value by using the output current setting signal 8. The output current detecting component 3b compares the reference voltage with the output current signal 4 to output a signal voltage. In addition, the above triangle wave and signal voltage are inputted to the output adjusting-signal generating component 3c, which compares them for adjustment to output an output adjusting signal 6 to the power conversion component 1.
With reference to FIG. 7 and FIG. 8, an explanation will be further made of the control of the output control component 3, FIG. 7 is a timing chart depicting generation of the output adjusting signal 6. An upper part of the figure depicts the triangle wave that is outputted from the frequency generating component 3a and the signal voltage that is outputted from the output current detecting component 3b. A lower part of the figure depicts the output adjusting signal 6 that is formed by adjusting the triangle wave and the signal voltage to be outputted in the output adjusting-signal generating component 3c. 
That is, the output adjusting-signal generating component 3c, when the output of the triangle wave exceeds the output of the adjusted output current signal, outputs the output adjusting signal 6 of Hi, and on the other hand, when the output of the triangle wave does not exceed the output of the signal voltage, outputs the output adjusting signal 6 of Low.
This output adjusting-signal generating component 3c functions in such a manner that, for example, in a case where the output current 5 is reduced to a value lower than a current defined for driving the LED 2, the output current signal 4 is compared with a reference voltage to be adjusted to be equal to the reference voltage. In this case, a ratio of the output adjusting signal 6 of Hi increases, and the output current 5 returns to the defined current. In addition, in a case where the output current 5 is increased to a value higher than the defined current, the output current signal 4 is compared with the reference voltage to be adjusted to be equal to the reference voltage. In this case, a ratio of the output adjusting signal 6 of Hi decreases, and the output current 5 returns to the defined current. It should be noted that the output adjusting-signal generating component 3c employs a circuit such as a comparator or a flip flop. In addition, a modulation method, for example, a pulse width modulation (PWM) is applied to the signal of the output adjusting signal 6.
FIG. 8(a) is a block circuit diagram of the power conversion component 1. As depicted in the figure, the power conversion component 1 has a predetermined, fixed inductance, and comprises a coil 20 in which a coil current IL flows, a switching element 30 such as a MosFET that converts power by a switching operation, an output detecting resistor 40 that is used for detection of the output current signal 4, an output detecting component 40a that amplifies the detected current to be outputted as the output current signal 4, a condenser 50, and a diode 60. The output adjusting signal 6 drives the switching element 30 in such a manner as to supply the set output current 5 to the LED 2. It should be noted that the power conversion component 1 comprising these components is a general pressure-increasing DC-DC converter, and the detailed explanation is omitted.
Next, an explanation will be made of FIG. 8(b). An upper part of the figure depicts a timing chart of Hi and Low as output of the output adjusting signal 6 depicted also in FIG. 7. A middle part of the figure depicts a timing chart of a value of the coil current IL flowing in the coil 20 in a case where the output current 5 is set as large. A lower part of the figure depicts a timing chart of a value of the coil current IL flowing in the coil 20 in a case where the output current 5 is set as small.
As depicted in FIG. 8(b), when the output adjusting signal 6 is Hi, the switching element is driven and the coil current IL flows to increase in the coil 20. That is, the coil current IL flows to increase when the output adjusting signal 6 is Hi and flows to decrease when the output adjusting signal 6 is Low. An inclination of the increase or decrease of the coil current IL is constant because the inductance as a constant of the coil 20 is fixed. Even if the inclination of the increase or decrease of the coil current IL is thus constant, in a case where the output current is set as large, the coil current IL is also large, and, as depicted in the middle part of FIG. 8(b), the coil current IL is not reduced to less than 0 A. In this case, the output current 5 is stably supplied to the LED 2. It should be noted that the inductance of the coil 20 is generally set to be stable when the output current is set as the maximum.
On the other hand, in a case where the output current is set as small, the coil current IL is also small, and, as depicted in the lower part of FIG. 8(b), the coil current IL is reduced to less than 0 A. In this case, the output current 5 becomes unstable, and therefore the unstable current is supplied to the LED 2. This is because, as described above, the frequency from the frequency generating component 3a is fixed and the frequency of the output adjusting signal 6 that drives the switching element 30 is fixed.
Japanese Unexamined Patent Application Publication No. 2011-172321 discloses a technology in regard to a vehicle lighting device provided with a pressure-increase/decrease DC-DC converter that increases or decreases an inputted direct-current voltage to be converted to an output voltage. The pressure-increase/decrease DC-DC converter comprises a pressure-increasing switch, a pressure-decreasing switch, a coil, and a control component.
The control component comprises a comparator component that compares an output value and a predetermined target value to output the comparison result, a triangle wave generating component that generates a triangle wave, and a drive pulse generating circuit that generates pulses for driving the pressure-increasing switch and the pressure-decreasing switch. The pulse is generated by inputting the calculation output voltage from the comparator component and the triangle wave from the triangle wave generating component to a comparator. That is, the drive pulse generating circuit outputs a low level signal in a case where a voltage value of the calculation output voltage is equal to or more than a voltage value of the triangle wave, and on the other hand, outputs a high level signal in a case where the voltage value of the calculation output voltage is less than the voltage value of the triangle wave.
In the conventional vehicle lighting device, in a case where a light emitting element that is employed as a load is an LED, a luminous flux thereof varies, and it is necessary to suppress the variation of the luminous flux. Therefore adjustment of the luminous flux, that is, a change in the current that is supplied to the LED is made, thus coping with the variation of the luminous flux. In addition, the LED is classified into a plurality of ranks corresponding to values of the luminous flux, and the current is set in accordance with the rank.
In the above conventional vehicle lighting device, the output current is changed by changing the setting of the resistance value or the reference voltage corresponding to each rank. However, the setting (duty) of ON or OFF for power control does not almost change in the set output current because of the current forward voltage characteristics of the LED. Accordingly the set output current requires a large width for being adapted to each rank of the vehicle lighting device.
In a case where the output current requires such a large width, a component such as a coli is set to be adapted to the maximum current value. Therefore there is a problem that when the set output current is low, the output current can not be maintained. This is the problem that, as explained with reference to FIG. 8(b), in a case where the output voltage does not almost change, that is, in a case where the duty does not change, the output current decreases, and therefore the limit value of the current flowing the coil is reduced to less than 0 A.
In this state, it is not possible to maintain the stable output current. In addition, in this state, it is required to change a constant of the circuit, for example, replace a component of the coil or the like for each set output current, or employ the circuit only in a narrow range of the output current in which the circuit can be adapted.
The vehicle lighting device provided with the pressure-increasing/decreasing DC-DC converter that is described in Japanese Unexamined Patent Application Publication No. 2011-172321, as described above, includes the drive pulse generating circuit in which the calculation output voltage from the comparator component and the triangle wave from the triangle wave generating component are inputted to the comparator to generate a drive pulse. However, the invention that is described in Japanese Unexamined Patent Application Publication No. 2011-172321 is designed to control the output voltage to be kept constant to the variation in input voltage, and is not configured to solve the foregoing problems.